In accordance with the prevalence of small sized electronic equipments, nickel-hydrogen batteries (Ni/MH batteries) in the first half of the 1990s, then lithium-ion batteries (LIBs) were commercialized successively as the power source in competition with the mainly used small sized nickel-cadmium batteries. As a result, currently, the latter LIBs have come to occupy a preponderant share in general use. Moreover, motive power use secondary batteries such as hybrid vehicles (HEV, P-HEV), electric vehicles (EV) aiming to solve global environmental issues and energy issues has begun to be put into practical use. Further, motive power use secondary batteries such as hybrid vehicles (HEV, P-HEV), electric vehicles (EV) aiming to solve global environmental issues and energy issues have begun to be put into practical use. Further, secondary battery market for industrial use such as energy storage, UPS are also predicted to grow massive hereafter, thus improvement and new development of electrochemical applied products such as medium/large sized secondary batteries and capacitors as the power source has become an urgent and important issue. For these secondary batteries as the main power source in power/industrial use, formation of the market has already begun by medium/large sized nickel-hydrogen batteries (Ni/MH batteries) and lead-acid batteries being put into practical use approximately 15 years ago. However, in the future, attention is drawn to nonaqueous batteries such as LIBs excellent in respects of small size, light weight, and high voltage as becoming mainly used secondary batteries hereafter in the further growing market. In addition, capacitors possible of ultra-rapid charge and discharge and with high response speed or lithium-ion capacitors (LICs) and the like has been drawn attention to as single or combined power source with secondary batteries such as LIBs.
Since the present invention relates to improving the method for manufacturing electrodes common to these electrochemical applied products, detailed explanation will be made hereinafter with respect to lithium-ion batteries (referred to as LIBs) for convenience of explanation.
The common issues to be improved in both general use, wherein small sized though high capacity LIBs have become necessary for use such as in smartphones, and medium/large sized LIBs for power/industrial use, wherein much large-scaled power source than general use and relatively high rate charge and discharge and long-term reliability in severe operating environments are necessary, in other words the strongest demands from the market regarding batteries are improvement in properties such as further high capacity and high reliability (including safety) (hereinafter referred to as “properties”) and cost reduction.
Because of this, designs for a power source system sufficiently considering high reliability from the step of battery designing are planned. Naturally, the following improvements for a primitive LIB itself have been performed from the past.
Development of a stable and low cost positive and negative electrode material contributing to “properties” improvements.
Development of a stable and low cost electrolyte, separator, and binder contributing to “properties” improvements.
Improvement in structure of the electrode and battery.
Improvement in the manufacturing process and improvement in quality control of the battery.
Consequently, currently, as the positive electrode material, other than Li compounds of Co, Mn, and Ni oxide which have been used from the past, Li compounds of iron phosphate in which low cost and heat stability are expected have begun to be put into practical use. Also, as the negative electrode material, other than carbonic materials which have been used from the past, lithium titanate (Li4Ti5O12) which is durable to long-term use and is excellent in rapid charge and discharge property and safety and the like have begun to be put into practical use. However, they both still have issues on reduction in energy density, that is, to achieve high capacity in “properties”. Moreover, though low cost is expected, when regarded as a finished battery, it has not yet made significant progress under the present circumstances.
As for the electrolyte, separator, and binder, improvement in high reliability of LIBs, such as improving heat resistance is made, but is basically related only to reliability in “properties”.
As to improvement in structure of the battery, improvement of reliability in “properties” has been planned by preventing micro short circuit and improving the discharging method of generated gas. However, significant progress in improvement in structure of the electrode is not recognized. Naturally, development of a thick positive and negative electrode greatly relating this argument seems to have been adopted, but when an active material is coated thickly to a metal foil, increase in electrode resistance as well as electrolyte shortage in the depth portion of the electrode occur and cause difficulty in rapid charge and discharge. As a result, significant practical application of a thick electrode is difficult.
Moreover, vigorous effort has been made to improvement in the manufacturing process and quality control of the battery by each battery manufacturers from the past and remarkable progress is seen to improvement and stabilization in battery quality, but this does not fundamentally contribute to significant improvement in “properties” and cost reduction.
However, focusing once again that the root cause hindering improvement in “properties” and cost reduction mentioned above is due to conventional thin electrodes (thickness: approximately 100 μm) designed in consideration of current collecting performance of the electrode or diffusion of the electrolyte, an LIB structured by a thick electrode employing a three-dimensional electrode substrate which improves current collecting performance of the whole intra-electrode has already been suggested, see: Japanese Patent No. 4536289, U.S. Pat. No. 6,800,399, Chinese Patent No. ZL201010582391.4.
In other words, it is a thick electrode employing a three-dimensional electrode substrate (hereinafter referred to as 3DF) which improves current collecting performance of the whole intra-electrode or diffusion of the electrolyte to the depth portion of the electrode instead of a thin electrode coating an active material and the like to a two-dimensional metal foil substrate. For example, in an LIB using a thick electrode of about two times than conventional, it can at least achieve high capacity for the reduced volume since the use area of the separator or the electrode substrate is reduced by half, and due to the space with the opposite electrode adjacent to an electrode lead being extended, risks of micro short circuit caused by vibration etc. can be reduced. Moreover, since the length of an electrode or the number of electrodes is reduced by half, manufacturing the battery becomes simple and the production quantity per unit time can be increased, so cost reduction can also be expected.
However, employing the conventional production method of electrodes in which a paste of an active material is coated to a metal foil substrate as it is, had the following problems.
1. The desired property could not be stably obtained since shortage in filling amount of the active material as well as disturbance of the Li ion movement were caused due to the air existing inside of the 3DF being irremovable.
2. It had the risk of causing micro short circuit of the battery due to Li dendrites (needle-like crystals) generated on the negative electrode surface at the end of charge by metallic powder dust generated by three-dimensional processing (3DF processing) of a metal foil or conductive dust in the air and the like adhered to the 3DF being mixed into to the paste side while coating the paste of the active material and a part of the 3DF being exposed to the electrode surface breaking through the separator.